1. Field of the Invention
The present invention relates to a process of producing aldehydes and, more particularly, it relates to a process of producing aldehydes by hydroformylating olefins in the presence of a rhodium-tertiary phosphine complex catalyst while maintaining the catalytic activity at a high level.
2. Description of the Prior Art
Recently, the hydroformylation of olefins using a soluble rhodium complex catalyst containing a tertiary phosphine as at least one of the ligands (hereinafter, for simplicity, the catalyst is referred to as a rhodium-tertiary phosphine complex catalyst) has been developed (e.g., as disclosed in U.S. Pat. No. 3,527,809), since the catalyst possesses a high catalytic activity, a quite high selectivity for aldehydes, in particular, for more valuable straight-chain aldehydes in the hydroformylation reaction of .alpha.-olefins, and a high stability which makes it possible to carry out the reaction under reduced pressure.
A rhodium-tertiary phosphine complex catalyst has an important advantage because of its high stability in that after separating the aldehyde thus formed the reaction product solution by distillation, the residual solution containing the rhodium-tertiary phosphine complex catalyst can be recycled to the reaction zone as a catalyst solution.
However, in the course of the repeated use of the recycled catalyst solution, high-boiling by-products and complexes which do not have any or have a reduced catalytic activity formed by a change in the structure of the rhodium-tertiary phosphine complex catalyst itself and by the action of impurities such as oxygen, halogens, sulfur, etc., contained to a small extent in the starting materials accumulate gradually in the catalyst solution.
Therefore, in order to carry out the hydroformylation reaction continuously and in a stable manner, a technique where the catalytic activity of the recyclic catalyst solution is maintained at a constant level by supplying fresh catalyst to the recycled solution and at the same time removing a part of the recycled catalyst solution must be employed.
In this case, however, recovery of rhodium from the catalyst solution thus removed from the recycling system is inevitable since rhodium is a quite expensive metal. However, the operation of recovering rhodium from the solution is complicated and this makes the hydroformylation reaction economically disadvantageous.